A brief History of Time pdfdrive com

particles of spin
3
/
2
, 1, ½, and 0. In a sense, all these particles could
then be regarded as different aspects of the same “superparticle,” thus
unifying the matter particles with spin ½ and
3
/
2
with the force-carrying
particles of spin 0, 1, and 2. The virtual particle/antiparticle pairs of spin
½ and
3
/
2
would have negative energy, and so would tend to cancel out
the positive energy of the spin 2, 1, and 0 virtual pairs. This would cause
many of the possible infinities to cancel out, but it was suspected that
some infinities might still remain. However, the calculations required to
find out whether or not there were any infinities left uncanceled were so
long and difficult that no one was prepared to undertake them. Even
with a computer it was reckoned it would take at least four years, and
the chances were very high that one would make at least one mistake,
probably more. So one would know one had the right answer only if
someone else repeated the calculation and got the same answer, and that
did not seem very likely!
Despite these problems, and the fact that the particles in the
supergravity theories did not seem to match the observed particles, most
scientists believed that supergravity was probably the right answer to
the problem of the unification of physics. It seemed the best way of
unifying gravity with the other forces. However, in 1984 there was a

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remarkable change of opinion in favor of what are called string theories.
In these theories the basic objects are not particles, which occupy a
single point of space, but things that have a length but no other
dimension, like an infinitely thin piece of string. These strings may have
ends (the so-called open strings) or they may be joined up with
themselves in closed loops (closed strings) (
Fig. 11.1
and
Fig. 11.2
). A